The electrical industry has been developed to one of the most important industries in the twentieth century. With the fast progressing in various kinds of computing and processing system, lots of electrical devices including computer, communication, and consumer electronics are employed in our daily life. In recent years, the application of image processing apparatus, in combining with the raising processing and storage ability of the computing system or the processors, plays an vital role in image capturing, processing, and transferring usage.
In the image capturing system like scanners and the image outputting system like printers, a driving apparatus is used for position control. Without limiting the scope of the present invention, a flatbed scanner is utilized as an illustrative example. Referring to FIG. 1, for a flatbed scanner to capture the image on a document, the document is placed still and a scanning head 10 is moved in the direction 12 to capture the image line-by-line. In traditional scanners or other kinds of image processing systems like printers, a stepping motor 14 is used as the driving apparatus for moving and position control. The stepping motor is designed to be driven by a driving or controlling system 16 in a step-by-step way and can be easily controlled with reduced cost in the simplified open-loop controlling system under low complexity.
In the application of the image processing, high accuracy is needed to achieve improved image quality. However, the conventional application of the stepping motor has several disadvantages. The cost of the stepping motor, which has a complex structure in the rotor or the stator, is higher than other kinds of motors, like DC motors. The high cost in the driving unit is hard to be lowered down especially on highly competitive products.
The step-by-step driving characteristics of the stepping motor is achieved by the design and wiring of coils on the rotor or the stator. The driving ability, namely the driving torque, is quite limited under the step-by-step operating mechanisms. If the friction of the whole system to be driven is too large, the stepping motor might miss or neglect the step under insufficient output torque. Thus the traditional stepping motor-driven system is found to have the problem of "missing steps". However, since the driving of the stepping motor is generally a one-way system without positional feedback, the steps missed are not identified by the system. With the unidentified missed steps, the captured image of the scanner might have repeated scanning lines in the middle of the document or some remained lines missed at the end of the document. The image on the document is thus scanned with above-identified errors and the correctness of the image is damaged.
Besides, under the design of the wired coils on the rotor or the stator, most stepping motors have limited accuracy of each stepped angle. The unequal stepped angles cause the moving of the image capturing head to have unequal steps or pitches. Some steps might be bigger than other steps, and the captured image is found to have a distortion problem. Since the problem is caused mainly by the stepping motor, the reduced accuracy of the image is hard to be compensated from modifying the system design or the processing software.
In addition to the above identified disadvantages, the problem of vibration and noise generated by the stepping motor also have negative effects. The accuracy, stability, and operation life of the system can be reduced under disturbing vibrations during the operating of the driving system. The noise problem further reduce the stability and competitive ability of the products.
In light of above, the stepping further has the problem of unable to operate at high speed. Therefore, an improved driving apparatus, especially a driving apparatus for the position control in the image processing system, is highly needed to improve the quality, correctness, and operating speed of the image processing or image capturing processes. What is needed is a driving system with simplified design and reduced cost to increase the stability and competitive ability of the products.